I'm not quite sure if this is possible or not, but I need to force a certain security group's users to have their passwords expire so they'll be forced to change them on next login. The reason for this is because I applied a FGPP (password policy) to this particular group in order to enforce strong passwords. Well, many users have really weak passwords and they won't be changed unless they're forced.

Is there a way to do this without forcing everyone to a single password?

You can combine this with other code to get the list users from the security group (use the security group CN= in the LDAP query) and loop through them applying this password expiration. This is a good example of that found by searching "vbscript getobject security group":
Get all users in specific AD group using VbScript